The selective oxidation of cycloaliphatic hydrocarbons to dicarboxylic acids, in particular, cyclohexane to adipic acid is indeed an industrially important reaction. The target product, adipic acid, is an extremely important commodity chemical for the manufacture of polyamides (e.g. nylon 6,6), polyurethanes, polyesters, plasticizers (e.g. PVC), and for the production of intermediates for pharmaceuticals, insecticides etc. Furthermore, it is also used in medicine and food industry for different applications.
The current production process of AA on a commercial scale involves two-steps. The first step deals with the oxidation of cyclohexane to produce mainly a mixture of cyclohexanone and cyclohexanol, the so-called KA-oil process (KA=Ketone and Alcohol) at around 150° C. and at 10-20 bar of air using a cobalt or a manganese catalyst. In the second step, the resulting mixture of KA is subsequently converted into AA using nitric acid as an oxidant. The majority of AA available in the market is made through KA-oil process according to the steps mentioned above. The main drawback of this method is the needed very low conversion levels of CH(X=≦5%) to reach high KA selectivity (70-85%). This requires large recycling (>90%) of un-reacted CH and hence incurs some additional costs. Another disadvantage of this method is concerning environmental issues. For instance, the usage of nitric acid as an oxidant in the second step of this process undeniably generates certain amounts of NOx in the product stream, which in turn needs some efforts to remove NOx from exhaust gases and not to liberate it to the atmosphere. Therefore, there is a need to look for other alternatives or develop new and attractive routes that can avoid usage of environmental unfriendly reagents. Various possible routes that can be used for producing adipic acid are known from the literature. Although different opportunities exist, the direct oxidation of cyclohexane to adipic acid in one step using molecular oxygen as an oxidant is indeed an effective and economic approach, which is also the main task of the present invention.
Even though some of the above processes are being practiced commercially, some process options for AA production in absence of HNO3 usage were also proposed by various research groups in different patents (e.g. GB 1304 855 (1973) and U.S. Pat. No. 3,390,174 (1968)). Nevertheless, these approaches gave only poor selectivities (S=30-50%) of desired products such as KA and/or adipic acid. An additional problem of most of these processes is usage of soluble homogeneous catalysts, which pose difficulty of separation after the reaction.
F. T. Starzyk et al. (Stud. Surf. Sci. Catal. 84 (1994) 1419) have applied “iron phthalocyanine encapsulated in Y-zeolite” as a solid catalyst. However, this process strongly suffers from much longer induction periods, i.e. the catalyst requires about 300 h to reach CH conversion of ca. 35% and needs 600 h to get higher amounts of adipic acid in the product stream, which makes the process commercially unattractive.
EP 519 569 (1992) report the use of Cobalt substituted ALPO-5 catalyst for the synthesis of AA by the oxidation of CH in acetic acid as a solvent. However, in this process, the possibility of formation of stable cyclohexylacetate as an additional by-product can not be ruled out because of the reaction between intermediate cyclohexanol and acetic acid. The formation of such unwanted by-product involves some additional separation costs. Furthermore, acetic acid is also corrosive solvent and hence difficulty of handling it in larger amounts on commercial scale and also special equipment is required for long-term operations dealing with acetic acid.
U.S. Pat. No. 6,392,093 (2002) discloses the use of a solid organotransition metal complex (e.g. encapsulated salen or substituted salen metal complex) for the oxidation of cyclohexane to adipic acid. Conversion levels of CH≦20% and the yield of AA below 10% are achieved. Another patent, EP 0784 045 B1 (2000) reported the use of metal incorporated tetra deca halo (nitro) phthalocyanine catalysts, where the metal=Co, Cu, Cr, Mn. They claimed the conversion of CH in the range from 6-15%, yield of AA=3-10% and the sum of yields of both cyclohexanone and cyclohexanol together 3-6%. In addition, WO 01/00555 A1 (2001) and U.S. Pat. No. 6,235,932 B1 (2001) describe a process for converting CH into AA employing the salts of Co or Co—Fe as catalysts mostly in acetate form. Although this patent claims high conversion of CH (13-55%) with good selectivity of AA (55-70%), these catalysts suffer from other problems such as an additional activation step prior to their use in the reactor, easy solubility of acetate salts (leaching) and the probability of formation of additional by-products (e.g. cyclohexyl acetate) can not be ruled out due to presence of acetates in reacting mixture. Moreover, such extra activation was done in an additional apparatus by bubbling oxygen through a solution of cobaltous-iron acetate in acetic acid at 90 to 130° C. in presence of methyl ethyl ketone or acetaldehyde etc. as promoters, which makes the approach complex.
Furthermore, efforts were also made by various researchers to use gold-based catalysts for the direct oxidation of CH to AA, but to the best of our knowledge all such attempts went unsuccessful until now. For instance, various gold catalysts such as Au/graphite (Y. J. Xu, P. Landon, D. Enache, A. F. Carley, M. W. Roberts, G. J. Hutchings, Catal. Lett. 101 (2005) 175.), Au/MCM-41 (G. Lu, R. Zhao, G. Qian, Y. Qi, X. Wang, J. Suo, Catal. Lett. 97 (2004) 115.), Au/SBA-15 (G. Lu, D. Ji, G. Qian, Y. Qi, X. Wang, J. Suo, Appl. Catal. A: Gen. 280 (2005) 175.), Au/CeO2 (A. Corma, J. Lopez Nieto, U.S. Pat. No. 7,166,751 (2007)), Au/SiO2 (L. X. Xu, C. H. He, M. Q. Zhu, K. J. Wu, Y. L. Xu, Catal. Lett. 118 (2007) 248.) and Au/Al2O3 (L. X. Xu, C. H. He, M. Q. Zhu, K. J. Wu, S. Fang, Catal. Lett. 114 (2002) 202.) were applied for the said reaction, which gave only cyclohexanol and cyclohexanone as major products without any adipic acid in the product stream. Using such catalyst systems, the conversion of CH was varied in the range from 6 to 20% but again almost no adipic acid formation was reported. However, the selectivity of both cyclohexanol and cyclohexanone products together were found to be in the range of 17 to 90%. Very recently, Hereijgers and Weckhuysen tried to use supported Au catalysts using various catalyst carries for the direct oxidation of CH to AA (B. P. C. Hereijgers, B. M. Weckhuysen, J. Catal. 270 (2010) 16.). However, these efforts again gave mainly cyclohexanone and cyclohexanol as main products (sum of selectivity of these two products are ca. 70% at a conversion of CH above 5%.
The object of the present invention is therefore to find effective and potential catalyst compositions for a direct method for producing dicarboxylic acids in a single step. The invention also aims to supply an easy method for preparing the catalyst and its use in the said oxidation reaction.
Especially it is an object of the present invention to provide a direct method for the preparation of adipic acid from cyclohexane in a single step.